FIG. 1 is a block diagram of a network structure of UMTS (universal mobile telecommunications system).
Referring to FIG. 1, a universal mobile telecommunications system (hereinafter abbreviated UMTS) mainly includes a user equipment (hereinafter abbreviated UE), a UMTS terrestrial radio access network (hereinafter abbreviated UTRAN) and a core network (hereinafter abbreviated CN).
The UTRAN includes at least one radio network sub-system (hereinafter abbreviated RNS). And, the RNS includes one radio network controller (hereinafter abbreviated RNC) and at least one base station (hereinafter called Node B) managed by the RNC. And, at least one or more cells exist in one Node B.
FIG. 2 is a diagram of architecture of UMTS radio protocol.
Referring to FIG. 2, radio protocol layers exist as pairs in both UE and UTRAN to take charge of data transmission in radio section.
The respective radio protocol layers are explained as follows.
First of all, a PHY layer as a first layer plays a role in transferring data to a radio section using various radio transfer techniques. In the PHY layer, a reliable data PHY layer of a radio section is connected to a MAC layer as an upper layer via a transport channel. And, the transport channel is mainly classified into a dedicated transport channel and a common transport channel according to whether a channel is shared or not.
A second layer includes MAC, RLC, PDCP and BMC layers. First of all, a MAC layer plays a role in mapping various logical channels to various transport channels, respectively and also performs a function of logical channel multiplexing that plays a role in, mapping various logical channels to one transport channel. The MAC layer is connected to an RLC layer of an upper layer via a logical channel.
And, the logical channel is mainly divided into a control channel for transferring information of a control plane and a traffic channel for transferring information of a user plane according to a type of information that is transferred.
Meanwhile, the MAC layer can be divided into MAC-b sublayer, MAC-d sublayer, MAC-c/sh sublayer and MAC-e sublayer according to types of transport channels managed in detail.
The MAC-b layer takes charge of a management of a transport channel BCH (broadcast channel) responsible for a broadcast of system information. The MAC-c/sh layer manages a shared transport channel, which is shared by other UEs, such as FACH (forward access channel), DSCH (downlink shared channel) and the like. The MAC-d sublayer takes charge of a management of a dedicated transport channel DCH (dedicated channel) for a specific UE. The MAC-hs sublayer manages a transport channel HS-DSCH (high speed downlink shared channel) for high speed data transfer to support the high speed data transfer in downlink and uplink. And, the MAC-e sublayer manages a transport channel E-DCH (enhanced dedicated channel) for uplink data transfer.
A radio link control (hereinafter abbreviated ‘RLC’) layer takes charge of guarantee of quality of service (hereinafter abbreviated ‘QoS’) of each radio bearer and also takes charge of a transfer of corresponding data. The RLC leaves one independent RLC entity at each RB to guarantee intrinsic QoS of RB. The RLC offers three kinds of RLC modes such as transparent mode (hereinafter abbreviated ‘TM’), unacknowledged mode (hereinafter abbreviated ‘UM’) and acknowledged mode (hereinafter abbreviated ‘AM’) to support various QoS. And, the RLC plays a role in adjusting a data size to enable a lower layer to transfer data to a radio section. For this, the RLC plays a role in segmenting and concatenating data received from an upper layer.
A PDCP layer is placed above the RLC layer and plays a role in transferring data transferred using IP packet such as IPv4 or IPv6 efficiently in a radio section having a relatively small bandwidth. For this, the PDCP layer performs a header compression function, by which information mandatory for a header of data is transferred to raise transport efficiency in a radio section. Since header compression is a basic function of the PDCP layer, the PDCP layer exists in a packet service domain (hereinafter abbreviated ‘PS domain’) only. And, one PDCP entity exists for each RB to provide an effective header compression function to each PS service.
In the second layer, a BMC (broadcast/multicast control) layer is provided above the RLC layer. The BMC layer schedules a cell broadcast message and performs broadcasting to UEs located in a specific cell.
A radio resource control (hereinafter abbreviated ‘RRC’) layer located in a lowest part of a third layer is defined by a control plane only. The RRC layer controls parameters of the first and second layers to be associated with establishment, re-configuration and release of RBs and takes charge of controlling logical, transport and physical channels. In this case, the RB means a logical path provided by the first and second layers of a radio protocol for data transfer between UE and UTRAN. And, RB establishment means a process of regulating characteristics of a radio protocol layer and channel to offer a specific service and establishing specific parameters and operational methods.
The RLC layer is explained in detail as follows.
First of all, basic functions of the RLC layer are QoS guarantee of each RB and a corresponding data transfer. Since an RB service is a service that the second layer provides to an upper layer, the entire second layer has influence on QoS. And, influence of RLC is the greatest. The RLC leaves an independent RLC entity at each RB to guarantee the intrinsic QoS of RB and offers three kinds of RLC modes of TM, UM and AM. Since the three RLC modes differ from one another in the supported QoS, their operational methods are different from one another as well as their detailed functions. So, the RLC needs to be looked into according to its operational mode.
TM RLC is a mode that any overhead is not attached to RLC service data unit (hereinafter abbreviated ‘SDU’) delivered from a higher layer in configuring RLC protocol data unit (hereinafter abbreviated ‘PDU’). In particular, since RLC transmits SDU transparently, it is called TM RLC. Due to such characteristics, TM RLC plays the following roles in user and control planes as follows. In the user plane, since data processing time within RLC is short, TM RLC performs real-time circuit data transfer such as voice or streaming in a circuit service domain (hereinafter abbreviated ‘CS domain’). Meanwhile, in the control plane, since there is no overhead within RLC, RLC takes charge of transmission of RRC message from an unspecific UE in case of uplink or transmission of RRC messages broadcast from all UEs within a cell in case of downlink.
Unlike the transparent mode, a mode of adding an overhead in RLC is called a non-transparent mode which is classified into an unacknowledged mode (UM) having no acknowledgement for the transmitted data and an acknowledged mode (AM) having acknowledgement for the transmitted data. By attaching a PDU header including a sequence number (hereinafter abbreviated ‘SN’) to each PDU, UM RLC enables a receiving side to know which PDU is lost in the course of transmission.
Owing to this function, the UM RLC mainly performs transmission of real-time packet data such as broadcast/multicast data transmission, voice of PS domain (e.g., VoIP) and streaming in user plane or transmission of RRC message needing no acknowledgement among RRC messages transmitted to a specific UE or specific UE group within a cell in control plane.
AM RLC as one of the non-transparent modes configures PDU by attaching a PDU header including SN like UM RLC. Yet, the AM RLC differs from the UM RLC in that a receiving side makes acknowledgement to PDU transmitted by a transmitting side. The reason why the receiving side makes acknowledgement in the AM RLC is because the transmitting side can make a request for retransmission of PDU failing to be received by the transmitting side it self. And, this retransmission function is the most outstanding feature of the AM RLC. So, the object of the AM RLC is to guarantee error-free data transmission through retransmission. Owing to this object, the AM RLC mainly takes charge of transmission of non-real-time packet data such as TCP/IP of PS domain in user plane or transmission of an acknowledgement-mandatory RRC message among RRC messages transmitted to a specific UE within a cell in control plane.
In aspect of directionality, TM or UM RLC is used for uni-directional communications, whereas AM RLC is used for bi-directional communications due to feedback from a receiving side. Since the bi-directional communications are mainly used for point-to-point communications, AM RLC uses a dedicated logical channel only. There exists a difference in structural aspect as follows. One RLC entity includes a transmission or reception structure in TM or UM RLC, whereas a transmitting side and a receiving side exists within one RLC entity in AM RLC.
The complexity of AM RLC is attributed to the retransmission function. The AM RLC includes a retransmission buffer for retransmission management as well as a transmitting/receiving buffer and performs various functions of use of a transmitting/receiving window for flow control, polling that a transmitting side requests status information from a receiving side of a peer RLC entity, a status report that a receiving side reports its buffer status to a transmitting side of a peer RLC entity, status PDU for carrying status information, piggyback of inserting status PDU in data PDU to raise data transfer efficiency and the like.
Meanwhile, there is a reset PDU making a request for resetting of all operations and parameters to an AM RLC entity of the other side in case that an AM RLC entity discovers a critical error in the course of operation. And, there is also a reset Ack PDU used for a response to the reset PDU. To support these functions, AM RLC needs various protocol parameters, status variables and timer. PDU used for data transfer control in status information report, status PDU, reset PDU or the like is called control PDU and PDU used for delivery of user data is called data PDU.
In brief, PDUs used by AM RLC can be mainly classified into two types. A first type is data PDU and a second type is control PDU. And, the control PDU includes status PDU, piggybacked status PDU, reset PDU and reset Ack PDU.
One of the cases of using control PDU is a reset procedure. The reset procedure is used in solving the error situation in operation of AM RLC. For instance of the error situation, sequence numbers mutually used are different from each other or PDU or SDU fails in transmissions amounting to a count limit. Through the reset procedure, AM RLC of a receiving side and AM RLC of a transmitting side reset environmental variables and then re-enter a status enabling communications.
The reset procedure is explained as follows.
First of all, a side having decided to initiate a reset procedure, i.e., AM RLC of a transmitting side includes a currently used transmitting direction hyper frame number (hereinafter abbreviated ‘HFN’) value in reset PDU and then transmits the reset PDU to a receiving side. AM RLC of the receiving side having received the reset PDU re-establishes an HFN value of its receiving direction and then resets environmental variables such as a sequence number and the like. Subsequently, the AM RLC of the receiving side includes its transmitting direction HFN in reset Ack PDU and then transmits the reset Ack PDU to the AM RLC of the transmitting side. If receiving the reset Ack PDU, the AM RLC of the transmitting side re-established its receiving direction HFN value and then resets environmental variables.
A structure of RLC PDU used by AM RLC entity is explained as follows.
FIG. 3 is a structural diagram of AM RLC PDU.
Referring to FIG. 3, AM RLC PDU is used when AM RLC entity attempts to transmit user data or piggybacked status information and polling bit. A user data part is configured as an 8-bit integer multiplication and a header of AM RLC PDU is constructed with a 2-octet sequence number. And, a header part of AM RLC PDU includes a length indicator.
FIG. 4 is a structural diagram of status PDU.
Referring to FIG. 4, status PDU includes different types of SUFIs (super fields). A size of the status PDU is variable but is limited to a size of a largest RLC PDU of a logical channel carrying the status PDU. In this case, the SUFI plays a role in notifying information indicating what kind of AM RLC PDU arrives at a receiving side or what kind of AM RLC PDU does not arrive at the receiving side, etc. The SUFI is constructed with three parts of a type, a length and a value.
FIG. 5 is a structural diagram of piggybacked status PDU.
Referring to FIG. 5, a structure of a piggybacked status PDU is similar to that of a status PDU but differs in that a D/C filed is replaced by a reserved bit (R2). The piggybacked status PDU is inserted in case that a sufficient space remains in AM RLC PDU. And, a PDU type value can be always fixed to ‘000’.
FIG. 6 is a structural diagram of reset ACK PDU.
Referring to FIG. 6, a reset PDU includes a sequence number named 1-bit RSN. And, a reset ACK PDU is transmitted in response to a received reset PDU in a manner of including RSN contained in the received reset PDU.
Parameters used for the PDU format are explained as follows.
First of all, a value of ‘D/C field’ indicates whether a corresponding PDU is a control PDU or a data PDU.
‘PDU Type’ indicates a type of the control PDU. In particular, ‘PDU Type’ indicates whether a corresponding PDU is a reset PDU or a status PDU, and the like.
‘Sequence Number’ value means sequence number information of AM RLC PDU.
Meanwhile, ‘Polling Bit’ value is set when a request for status report is made to a receiving side.
Extension bit (E)’ value indicates whether a next octet is a length indicator or not.
Reserved bit (R1)’ value is used for a reset PDU or a reset ACK PDU and is coded as ‘000’.
‘Header Extension Bit (HE)’ value indicates whether a next octet is a length indicator or data.
‘Length Indicator’ value indicates a location of a boundary face if a boundary surface between different SDUs exists within a data part of PDU.
‘PAD’ part is a padding area and is an area that is not used in AM RLC PDU.
SUFI (Super Field) is explained in detail as follows.
As briefly mentioned in the foregoing description, SUFI plays a role in notifying information, which indicates what kind of AM RLC PDU has arrived at a receiving side or what kind of AM RLC PDU has not arrived at the receiving side and the like, to a transmitting side. Currently, there are eight types of SUFIs defined to use. Each of the SUFIs consists of a type, a length and a value.
And, there exist various SUFI types including NO_MORE (No More Data), WINDOW (Window Size), ACK (Acknowledgement), LIST (List), BITMAP (Bitmap), Rlist (Relative list), MRW (Move Receiving Window), MRW ACK (Move Receiving Window Acknowledgement), etc.
The SUFI types are explained in detail as follows.
(A) NO_MORE SUFI
FIG. 7 is a structural diagram of NO_MORE SUFI field according to a related art.
Referring to FIG. 7, NO_MORE SUFI exists as a type field only. NO_MORE SUFI plays a role in indicating that no more SUFI exists after the NO_MORE SUFI. So, an area following the SUFI can be regarded as PAD (padding) area.
(B) BITMAP SUFI
FIG. 8 is a structural diagram of BITMAP SUFI field according to a related art.
Referring to FIG. 8, BITMAP SUFI consists of a type (Type), a bitmap length (LENGTH), a start sequence number (F SN) and a bitmap (Bitmap).
The LENGTH consists of four bits and (LENGTH+1) means an octet size of the bitmap. For instance, if LENGTH=‘0000’, it means that a bitmap octet size is ‘1’. Since LENGTH can have set to a value up to ‘1111’, a maximum octet size the bitmap can have becomes ‘16’.
The FSN consists of twelve bits and means a sequence number corresponding to a first bit of the bitmap.
The Bitmap varies according to a value given by the Length field. Status information of AM RLC PDU corresponding to a sequence number in an interval corresponding to [FSN, FSN+(LENGTH+1)*8−1] can be indicated. In a sequence, a sequence number increases left to right and a reception status of AM RLC PDU is represented as ‘0’ (abnormal reception: NACI) or ‘1’ (normal reception: ACK).
In UE, AM RLC PDUs reported by BITMAP SUFI as correctly received a receiving side can be deleted by a transmitting side.
(C) ACK SUFI
FIG. 9 is a structural diagram of ACK SUFI field according to a related art.
Referring to FIG. 9, ACK SUFI consists of a type (Type) and a last sequence number (LSN).
ACK SUFI plays a role in indicating a last portion of a data part in STATUS PDU like the NO_MORE SUFI. If ACK SUFI exists at a last portion of STATUS PDU, NO_MORE SUFI needs not to exist at the same time. In other words, ACK SUFI should exist in STATUS PDU not ended as NO_MORE SUFI. Portions following the ACK SUFI can be regarded as PAD (padding).
The ACK SUFI takes charge of ‘acknowledge’ for reception of all AM RLC PDUs each of which is reported errorless in portions prior to a STATUS PDU in case of SN<LSN. In other words, if LSN>VR(R), acknowledgement information for AM RLC PDUs in reception error status should be transmitted as one using one STATUS PDU. In particular, acknowledgement information for AM RLC PDUs in reception error status are unable to be transmitted by being separated into several STATUS PDUs. If LSN=VR(R), AM RLC PDUs in reception error status can be transmitted by being separated into several STATUS PDUs. If LSN<VR(R), it cannot be used. And, a value of LSN can be set to a value equal to or smaller than VR(H). In this case, VR(H) is SN of AM RLC PDU that will arrive after a greatest SN among AM RLC PDUs received by a receiving side. In particular, in case of receiving ‘x’ having a greatest SN among AM RLC PDUs received by a receiving side, VR(H) becomes (x+1).
A transmitting side having received STATUS PDU can update a value of VT(A) by comparing LSN to SN of AM RLC PDU in a first error reception status included in the STATUS PDU.
If a value of the received LSN is equal to or smaller than the SN of the AM RLC PDU corresponding to a first reception error in STATUS PDU (LSN≦the SN of the first error bit in STATUS PDU), VT(A) is updated into the LSN value.
If a value of the received LSN is greater than the SN of the AM RLC PDU corresponding to a first reception error in STATUS PDU (LSN>the SN of the first error bit in STATUS PDU), VT(A) is updated into a value of SN of the AM RLC PDU corresponding to the first reception error in the STATUS PDU.
VR(R) is SN of AM RLC PDU estimated to be received next to a last AM RLC PDU received in sequence by the receiving side. For instance, if the receiving side receives AM RLC PDUs up to Nth AM RLC PDU without reception error, VR(R) is (N+1). And, VT(A) is SN of AM RLC PDU estimated to be received in sequence next to last AM RLC PDU having received ACK (normal reception acknowledgement) information from the receiving side by the transmitting side. For instance, if ACK (normal reception acknowledgement) information for AM RLC PDUs up to the Mth is received from the receiving side by the transmitting side, VT(A) is (M+1).
In the related art, if a transmitting side receives a status report, a lower edge of a transmission window is updated with reference to an LSN value included in an ACK SUFI. Namely, in the related art, an AM RLC of a transmitting side is supposed to update a value of VT(A) of a lower edge of the transmission window by comparing the received LSN value to a sequence number (SN) of AM RLC PDU reported as a reception error in status PDU or Piggybacked status PDU for status report.
However, in the related art, a transmitting side is unable to update a transmission window until receiving an ACK SUFI including an LSN value from a receiving side. In case that a status PDU or piggybacked status PDU including an ACK SUFI is not normally received, it is unable to update a transmission window despite recognizing that a receiving side has received AM RLC PDUs without error. Hence, it is unable to efficiently perform data transmission to the receiving side.